Determining a pulse timing function for a linear printhead

ABSTRACT

A method is described for controlling a printhead including an array of light sources. An aim exposure function is provided which gives an aim exposure as a function of an integer pulse count. An initial pulse timing function is also provided which defines an exposure time as a function of pulse count. A light output function for the light sources is determined responsive to the pulse timing function, wherein the light output function gives a light output of the light sources as a function of exposure time. The pulse timing function is updated responsive to the light output function and the aim exposure function. The process is repeated until a predefined iteration termination criterion is satisfied. The determined pulse timing function is used to control the printhead, wherein each light source is activated for a pulse count corresponding to a pixel code value of an associated image pixel.

CROSS-REFERENCE TO RELATED APPLICATIONS

Reference is made to commonly assigned, co-pending U.S. patentapplication Ser. No. 15/635,520, entitled: “Adaptive printheadcalibration process,” by Kuo et al.; and to commonly assigned,co-pending U.S. patent application Ser. No. 15/635,596, entitled: “Printengine with print-mode-dependent pulse timing functions,” by Kuo et al.,each of which is incorporated herein by reference.

FIELD OF THE INVENTION

This invention pertains to the field of electrographic printing and moreparticularly to a method for determining a pulse timing function forcontrolling a linear printhead.

BACKGROUND OF THE INVENTION

Electrophotography is a useful process for printing images on a receiver(or “imaging substrate”), such as a piece or sheet of paper or anotherplanar medium (e.g., glass, fabric, metal, or other objects) as will bedescribed below. In this process, an electrostatic latent image isformed on a photoreceptor by uniformly charging the photoreceptor andthen discharging selected areas of the uniform charge to yield anelectrostatic charge pattern corresponding to the desired image (i.e., a“latent image”).

After the latent image is formed, charged toner particles are broughtinto the vicinity of the photoreceptor and are attracted to the latentimage to develop the latent image into a toner image. Note that thetoner image may not be visible to the naked eye depending on thecomposition of the toner particles (e.g., clear toner).

After the latent image is developed into a toner image on thephotoreceptor, a suitable receiver is brought into juxtaposition withthe toner image. A suitable electric field is applied to transfer thetoner particles of the toner image to the receiver to form the desiredprint image on the receiver. The imaging process is typically repeatedmany times with reusable photoreceptors.

The receiver is then removed from its operative association with thephotoreceptor and subjected to heat or pressure to permanently fix(i.e., “fuse”) the print image to the receiver. Plural print images(e.g., separation images of different colors) can be overlaid on thereceiver before fusing to form a multicolor print image on the receiver.

Typically, a linear printhead including an array of LED light sources isused to form the electrostatic latent image. Differences between thepower provided by the individual light sources can result in streakartifacts being formed in the printed image. Even if the printhead iscarefully calibrated in the factory to equalize the power provided byeach light source, it has been found that when the printhead isinstalled into a printing system that there can be residual streakartifacts, and that these artifacts can change with time. Therefore,there remains a need for a method that can be performed in the field tocalibrate the printhead to equalize the exposure provided by each lightsource.

Typically, a linear printhead including an array of LED light sources isused to form the electrostatic latent image. The printhead generally hasan 8-bit interface which enables 256 different exposure levels to beprovided by each of the light sources. The exposure level provided bythe light sources is typically controlled by adjusting a time that thelight sources are activated, where each of the pixel code values ismapped to an exposure time that provides an aim exposure level.

To control the exposure time, some printheads utilize a stream ofexposure clock pulses having non-uniform pulse widths, where the pulsewidths are selected to provide the aim exposure levels. The exposuretime for a particular pixel is controlled by counting a number ofexposure clock pulses corresponding to the pixel code value. Forexample, for a pixel code value of 100, the light source would beactivated for 100 exposure clock pulses. However, it has been found thatthe optical power provided by the light sources is not constant withtime, and that the shape of the light output function is a function ofthe pulse widths of the exposure clock pulses. Therefore, determiningthe pulses widths required to provide the desired aim exposures can be acomplex process because changing the pulse width to modify the exposuretime changes the power, which will in turn affect the exposure timerequired to provide the aim exposure. In some printing systems, the aimexposure level as a function of pixel code value may be updated in thefield as part of a printer calibration process. It is thereforenecessary to update the pulse widths of the exposure clock pulsesaccordingly. There remains a need for an efficient method fordetermining a pulse timing function that can be implemented in thefield, and for controlling the printer with a pulse timing functionappropriate for a particular print mode.

SUMMARY OF THE INVENTION

The present invention represents a method for controlling a printhead ina digital printing system, the printhead including an array of lightsources for exposing a photosensitive medium, including:

a) providing an aim exposure function which gives an aim exposure to beprovided by the light sources as a function of an integer pulse count;

b) providing an initial pulse timing function which defines an exposuretime as a function of pulse count;

c) determining a light output function for the light sources responsiveto the pulse timing function, wherein the light output function gives alight output of the light sources as a function of exposure time;

d) updating the pulse timing function responsive to the light outputfunction and the aim exposure function;

e) repeating steps c)-d) until a predefined iteration terminationcriterion is satisfied; and

f) using the pulse timing function to control the printhead, whereineach light source is activated for a pulse count corresponding to apixel code value of an associated image pixel.

This invention has the advantage that a pulse timing function can bedetermined that will provide a specified aim exposure function forprintheads where the light output function varies with changes in thepulse timing function.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational cross-section of an electrophotographic printersuitable for use with various embodiments;

FIG. 2 is an elevational cross-section of one printing module of theelectrophotographic printer of FIG. 1;

FIG. 3 shows a processing path for producing a printed image using apre-processing system couple to a print engine;

FIG. 4 is a flow chart showing processing operations that are used toapply various calibration and artifact correction processes inaccordance with exemplary embodiments;

FIG. 5 illustrates an exemplary quantization look-up-table;

FIG. 6 illustrates an exemplary aim exposure function;

FIG. 7 is a graph illustrating how the master clock signal and theexposure clock signal are used to control the activation of a lightsource;

FIG. 8 is a flow chart of an iterative process for determining a pulsetiming function in accordance with an exemplary embodiment;

FIG. 9A compares an initial pulse timing function and an updated pulsetiming function;

FIG. 9B compares an initial light output function and an updated lighttiming function corresponding to the pulse timing functions of FIG. 9A;

FIG. 10 is a flow chart of a process for determining current controlparameters in accordance with an exemplary embodiment;

FIG. 11 shows an exemplary test target for use with the process of FIG.10;

FIG. 12 is a flow chart showing additional details of the analyzecaptured image step of FIG. 10;

FIG. 13 shows an exemplary set of measured test patch data;

FIG. 14 shows an exemplary calibration function relating scanner codevalues to estimated exposure values;

FIG. 15 a graph showing the estimated exposure error as a function oflight source for a particular test patch;

FIG. 16 is a graph showing the estimated exposure error for a particularlight source;

FIG. 17 is a graph illustrating an exemplary set of gain corrections;

FIG. 18 illustrates an exemplary user interface that enables a user toselect options for specifying a print mode;

FIG. 19 shows a processing path including an print engine that isadapted to produce printed images from image data using a plurality ofprint modes; and

FIG. 20 illustrates an exemplary set of pulse timing functionsappropriate for use with different print modes.

It is to be understood that the attached drawings are for purposes ofillustrating the concepts of the invention and may not be to scale.Identical reference numerals have been used, where possible, todesignate identical features that are common to the figures.

DETAILED DESCRIPTION OF THE INVENTION

The invention is inclusive of combinations of the embodiments describedherein. References to “a particular embodiment” and the like refer tofeatures that are present in at least one embodiment of the invention.Separate references to “an embodiment” or “particular embodiments” orthe like do not necessarily refer to the same embodiment or embodiments;however, such embodiments are not mutually exclusive, unless soindicated, or as are readily apparent to one of skill in the art. Theuse of singular or plural in referring to the “method” or “methods” andthe like is not limiting. It should be noted that, unless otherwiseexplicitly noted or required by context, the word “or” is used in thisdisclosure in a non-exclusive sense.

As used herein, “sheet” is a discrete piece of media, such as receivermedia for an electrophotographic printer (described below). Sheets havea length and a width. Sheets are folded along fold axes (e.g.,positioned in the center of the sheet in the length dimension, andextending the full width of the sheet). The folded sheet contains two“leaves,” each leaf being that portion of the sheet on one side of thefold axis. The two sides of each leaf are referred to as “pages.” “Face”refers to one side of the sheet, whether before or after folding.

As used herein, “toner particles” are particles of one or morematerial(s) that are transferred by an electrophotographic (EP) printerto a receiver to produce a desired effect or structure (e.g., a printimage, texture, pattern, or coating) on the receiver. Toner particlescan be ground from larger solids, or chemically prepared (e.g.,precipitated from a solution of a pigment and a dispersant using anorganic solvent), as is known in the art. Toner particles can have arange of diameters (e.g., less than 8 μm, on the order of 10-15 μm, upto approximately 30 μm, or larger), where “diameter” preferably refersto the volume-weighted median diameter, as determined by a device suchas a Coulter Multisizer. When practicing this invention, it ispreferable to use larger toner particles (i.e., those having diametersof at least 20 μm) in order to obtain the desirable toner stack heightsthat would enable macroscopic toner relief structures to be formed.

“Toner” refers to a material or mixture that contains toner particles,and that can be used to form an image, pattern, or coating whendeposited on an imaging member including a photoreceptor, aphotoconductor, or an electrostatically-charged or magnetic surface.Toner can be transferred from the imaging member to a receiver. Toner isalso referred to in the art as marking particles, dry ink, or developer,but note that herein “developer” is used differently, as describedbelow. Toner can be a dry mixture of particles or a suspension ofparticles in a liquid toner base.

As mentioned already, toner includes toner particles; it can alsoinclude other types of particles. The particles in toner can be ofvarious types and have various properties. Such properties can includeabsorption of incident electromagnetic radiation (e.g., particlescontaining colorants such as dyes or pigments), absorption of moistureor gasses (e.g., desiccants or getters), suppression of bacterial growth(e.g., biocides, particularly useful in liquid-toner systems), adhesionto the receiver (e.g., binders), electrical conductivity or low magneticreluctance (e.g., metal particles), electrical resistivity, texture,gloss, magnetic remanence, florescence, resistance to etchants, andother properties of additives known in the art.

In single-component or mono-component development systems, “developer”refers to toner alone. In these systems, none, some, or all of theparticles in the toner can themselves be magnetic. However, developer ina mono-component system does not include magnetic carrier particles. Indual-component, two-component, or multi-component development systems,“developer” refers to a mixture including toner particles and magneticcarrier particles, which can be electrically-conductive or-non-conductive. Toner particles can be magnetic or non-magnetic. Thecarrier particles can be larger than the toner particles (e.g., 15-μm or20-300 μm in diameter). A magnetic field is used to move the developerin these systems by exerting a force on the magnetic carrier particles.The developer is moved into proximity with an imaging member or transfermember by the magnetic field, and the toner or toner particles in thedeveloper are transferred from the developer to the member by anelectric field, as will be described further below. The magnetic carrierparticles are not intentionally deposited on the member by action of theelectric field; only the toner is intentionally deposited. However,magnetic carrier particles, and other particles in the toner ordeveloper, can be unintentionally transferred to an imaging member.Developer can include other additives known in the art, such as thoselisted above for toner. Toner and carrier particles can be substantiallyspherical or non-spherical.

The electrophotographic process can be embodied in devices includingprinters, copiers, scanners, and facsimiles, and analog or digitaldevices, all of which are referred to herein as “printers.” Variousembodiments described herein are useful with electrostatographicprinters such as electrophotographic printers that employ tonerdeveloped on an electrophotographic receiver, and ionographic printersand copiers that do not rely upon an electrophotographic receiver.Electrophotography and ionography are types of electrostatography(printing using electrostatic fields), which is a subset ofelectrography (printing using electric fields). The present inventioncan be practiced using any type of electrographic printing system,including electrophotographic and ionographic printers.

A digital reproduction printing system (“printer”) typically includes adigital front-end processor (DFE), a print engine (also referred to inthe art as a “marking engine”) for applying toner to the receiver, andone or more post-printing finishing system(s) (e.g., a UV coatingsystem, a glosser system, or a laminator system). A printer canreproduce pleasing black-and-white or color images onto a receiver. Aprinter can also produce selected patterns of toner on a receiver, whichpatterns (e.g., surface textures) do not correspond directly to avisible image.

In an embodiment of an electrophotographic modular printing machineuseful with various embodiments (e.g., the NEXPRESS SX 3900 printermanufactured by Eastman Kodak Company of Rochester, N.Y.) color-tonerprint images are made in a plurality of color imaging modules arrangedin tandem, and the print images are successively electrostaticallytransferred to a receiver adhered to a transport web moving through themodules. Colored toners include colorants, (e.g., dyes or pigments)which absorb specific wavelengths of visible light. Commercial machinesof this type typically employ intermediate transfer members in therespective modules for transferring visible images from thephotoreceptor and transferring print images to the receiver. In otherelectrophotographic printers, each visible image is directly transferredto a receiver to form the corresponding print image.

Electrophotographic printers having the capability to also deposit cleartoner using an additional imaging module are also known. The provisionof a clear-toner overcoat to a color print is desirable for providingfeatures such as protecting the print from fingerprints, reducingcertain visual artifacts or providing desired texture or surface finishcharacteristics. Clear toner uses particles that are similar to thetoner particles of the color development stations but without coloredmaterial (e.g., dye or pigment) incorporated into the toner particles.However, a clear-toner overcoat can add cost and reduce color gamut ofthe print; thus, it is desirable to provide for operator/user selectionto determine whether or not a clear-toner overcoat will be applied tothe entire print. A uniform layer of clear toner can be provided. Alayer that varies inversely according to heights of the toner stacks canalso be used to establish level toner stack heights. The respectivecolor toners are deposited one upon the other at respective locations onthe receiver and the height of a respective color toner stack is the sumof the toner heights of each respective color. Uniform stack heightprovides the print with a more even or uniform gloss.

FIGS. 1 and 2 are elevational cross-sections showing portions of atypical electrophotographic printer 100 useful with various embodiments.Printer 100 is adapted to produce images, such as single-color images(i.e., monochrome images), or multicolor images such as CMYK, orpentachrome (five-color) images, on a receiver. Multicolor images arealso known as “multi-component” images. One embodiment involves printingusing an electrophotographic print engine having five sets ofsingle-color image-producing or image-printing stations or modulesarranged in tandem, but more or less than five colors can be combined ona single receiver. Other electrophotographic writers or printerapparatus can also be included. Various components of printer 100 areshown as rollers; other configurations are also possible, includingbelts.

Referring to FIG. 1, printer 100 is an electrophotographic printingapparatus having a number of tandemly-arranged electrophotographicimage-forming printing subsystems 31, 32, 33, 34, 35, also known aselectrophotographic imaging subsystems. Each printing subsystem 31, 32,33, 34, 35 produces a single-color toner image for transfer using arespective transfer subsystem 50 (for clarity, only one is labeled) to areceiver 42 successively moved through the modules. In some embodimentsone or more of the printing subsystem 31, 32, 33, 34, 35 can print acolorless toner image, which can be used to provide a protectiveovercoat or tactile image features. Receiver 42 is transported fromsupply unit 40, which can include active feeding subsystems as known inthe art, into printer 100 using a transport web 81. In variousembodiments, the visible image can be transferred directly from animaging roller to a receiver, or from an imaging roller to one or moretransfer roller(s) or belt(s) in sequence in transfer subsystem 50, andthen to receiver 42. Receiver 42 is, for example, a selected section ofa web or a cut sheet of a planar receiver media such as paper ortransparency film.

In the illustrated embodiments, each receiver 42 can have up to fivesingle-color toner images transferred in registration thereon during asingle pass through the five printing subsystems 31, 32, 33, 34, 35 toform a pentachrome image. As used herein, the term “pentachrome” impliesthat in a print image, combinations of various of the five colors arecombined to form other colors on the receiver at various locations onthe receiver, and that all five colors participate to form processcolors in at least some of the subsets. That is, each of the five colorsof toner can be combined with toner of one or more of the other colorsat a particular location on the receiver to form a color different thanthe colors of the toners combined at that location. In an exemplaryembodiment, printing subsystem 31 forms black (K) print images, printingsubsystem 32 forms yellow (Y) print images, printing subsystem 33 formsmagenta (M) print images, and printing subsystem 34 forms cyan (C) printimages.

Printing subsystem 35 can form a red, blue, green, or other fifth printimage, including an image formed from a clear toner (e.g., one lackingpigment). The four subtractive primary colors, cyan, magenta, yellow,and black, can be combined in various combinations of subsets thereof toform a representative spectrum of colors. The color gamut of a printer(i.e., the range of colors that can be produced by the printer) isdependent upon the materials used and the process used for forming thecolors. The fifth color can therefore be added to improve the colorgamut. In addition to adding to the color gamut, the fifth color canalso be a specialty color toner or spot color, such as for makingproprietary logos or colors that cannot be produced with only CMYKcolors (e.g., metallic, fluorescent, or pearlescent colors), or a cleartoner or tinted toner. Tinted toners absorb less light than theytransmit, but do contain pigments or dyes that move the hue of lightpassing through them towards the hue of the tint. For example, ablue-tinted toner coated on white paper will cause the white paper toappear light blue when viewed under white light, and will cause yellowsprinted under the blue-tinted toner to appear slightly greenish underwhite light.

Receiver 42 a is shown after passing through printing subsystem 31.Print image 38 on receiver 42 a includes unfused toner particles.Subsequent to transfer of the respective print images, overlaid inregistration, one from each of the respective printing subsystems 31,32, 33, 34, 35, receiver 42 a is advanced to a fuser module 60 (i.e., afusing or fixing assembly) to fuse the print image 38 to the receiver 42a. Transport web 81 transports the print-image-carrying receivers to thefuser module 60, which fixes the toner particles to the respectivereceivers, generally by the application of heat and pressure. Thereceivers are serially de-tacked from the transport web 81 to permitthem to feed cleanly into the fuser module 60. The transport web 81 isthen reconditioned for reuse at cleaning station 86 by cleaning andneutralizing the charges on the opposed surfaces of the transport web81. A mechanical cleaning station (not shown) for scraping or vacuumingtoner off transport web 81 can also be used independently or withcleaning station 86. The mechanical cleaning station can be disposedalong the transport web 81 before or after cleaning station 86 in thedirection of rotation of transport web 81.

In the illustrated embodiment, the fuser module 60 includes a heatedfusing roller 62 and an opposing pressure roller 64 that form a fusingnip 66 therebetween. In an embodiment, fuser module 60 also includes arelease fluid application substation 68 that applies release fluid,e.g., silicone oil, to fusing roller 62. Alternatively, wax-containingtoner can be used without applying release fluid to the fusing roller62. Other embodiments of fusers, both contact and non-contact, can beemployed. For example, solvent fixing uses solvents to soften the tonerparticles so they bond with the receiver. Photoflash fusing uses shortbursts of high-frequency electromagnetic radiation (e.g., ultravioletlight) to melt the toner. Radiant fixing uses lower-frequencyelectromagnetic radiation (e.g., infrared light) to more slowly melt thetoner. Microwave fixing uses electromagnetic radiation in the microwaverange to heat the receivers (primarily), thereby causing the tonerparticles to melt by heat conduction, so that the toner is fixed to thereceiver.

The fused receivers (e.g., receiver 42 b carrying fused image 39) aretransported in series from the fuser module 60 along a path either to anoutput tray 69, or back to printing subsystems 31, 32, 33, 34, 35 toform an image on the backside of the receiver (i.e., to form a duplexprint). Receivers 42 b can also be transported to any suitable outputaccessory. For example, an auxiliary fuser or glossing assembly canprovide a clear-toner overcoat. Printer 100 can also include multiplefuser modules 60 to support applications such as overprinting, as knownin the art.

In various embodiments, between the fuser module 60 and the output tray69, receiver 42 b passes through a finisher 70. Finisher 70 performsvarious paper-handling operations, such as folding, stapling,saddle-stitching, collating, and binding.

Printer 100 includes main printer apparatus logic and control unit (LCU)99, which receives input signals from various sensors associated withprinter 100 and sends control signals to various components of printer100. LCU 99 can include a microprocessor incorporating suitable look-uptables and control software executable by the LCU 99. It can alsoinclude a field-programmable gate array (FPGA), programmable logicdevice (PLD), programmable logic controller (PLC) (with a program in,e.g., ladder logic), microcontroller, or other digital control system.LCU 99 can include memory for storing control software and data. In someembodiments, sensors associated with the fuser module 60 provideappropriate signals to the LCU 99. In response to the sensor signals,the LCU 99 issues command and control signals that adjust the heat orpressure within fusing nip 66 and other operating parameters of fusermodule 60. This permits printer 100 to print on receivers of variousthicknesses and surface finishes, such as glossy or matte.

FIG. 2 shows additional details of printing subsystem 31, which isrepresentative of printing subsystems 32, 33, 34, and 35 (FIG. 1).Photoreceptor 206 of imaging member 111 includes a photoconductive layerformed on an electrically conductive substrate. The photoconductivelayer is an insulator in the substantial absence of light so thatelectric charges are retained on its surface. Upon exposure to light,the charge is dissipated. In various embodiments, photoreceptor 206 ispart of, or disposed over, the surface of imaging member 111, which canbe a plate, drum, or belt. Photoreceptors can include a homogeneouslayer of a single material such as vitreous selenium or a compositelayer containing a photoconductor and another material. Photoreceptors206 can also contain multiple layers.

Charging subsystem 210 applies a uniform electrostatic charge tophotoreceptor 206 of imaging member 111. In an exemplary embodiment,charging subsystem 210 includes a wire grid 213 having a selectedvoltage. Additional necessary components provided for control can beassembled about the various process elements of the respective printingsubsystems. Meter 211 measures the uniform electrostatic charge providedby charging subsystem 210.

An exposure subsystem 220 is provided for selectively modulating theuniform electrostatic charge on photoreceptor 206 in an image-wisefashion by exposing photoreceptor 206 to electromagnetic radiation toform a latent electrostatic image. The uniformly-charged photoreceptor206 is typically exposed to actinic radiation provided by selectivelyactivating particular light sources in an LED array or a laser deviceoutputting light directed onto photoreceptor 206. In embodiments usinglaser devices, a rotating polygon (not shown) is sometimes used to scanone or more laser beam(s) across the photoreceptor in the fast-scandirection. One pixel site is exposed at a time, and the intensity orduty cycle of the laser beam is varied at each dot site. In embodimentsusing an LED array, the array can include a plurality of LEDs arrangednext to each other in a linear array extending in a cross-trackdirection such that all dot sites in one row of dot sites on thephotoreceptor can be selectively exposed simultaneously, and theintensity or duty cycle of each LED can be varied within a line exposuretime to expose each pixel site in the row during that line exposuretime.

As used herein, an “engine pixel” is the smallest addressable unit onphotoreceptor 206 which the exposure subsystem 220 (e.g., the laser orthe LED) can expose with a selected exposure different from the exposureof another engine pixel. Engine pixels can overlap (e.g., to increaseaddressability in the slow-scan direction). Each engine pixel has acorresponding engine pixel location, and the exposure applied to theengine pixel location is described by an engine pixel level.

The exposure subsystem 220 can be a write-white or write-black system.In a write-white or “charged-area-development” system, the exposuredissipates charge on areas of photoreceptor 206 to which toner shouldnot adhere. Toner particles are charged to be attracted to the chargeremaining on photoreceptor 206. The exposed areas therefore correspondto white areas of a printed page. In a write-black or “discharged-areadevelopment” system, the toner is charged to be attracted to a biasvoltage applied to photoreceptor 206 and repelled from the charge onphotoreceptor 206. Therefore, toner adheres to areas where the charge onphotoreceptor 206 has been dissipated by exposure. The exposed areastherefore correspond to black areas of a printed page.

In the illustrated embodiment, meter 212 is provided to measure thepost-exposure surface potential within a patch area of a latent imageformed from time to time in a non-image area on photoreceptor 206. Othermeters and components can also be included (not shown).

A development station 225 includes toning shell 226, which can berotating or stationary, for applying toner of a selected color to thelatent image on photoreceptor 206 to produce a developed image onphotoreceptor 206 corresponding to the color of toner deposited at thisprinting subsystem 31. Development station 225 is electrically biased bya suitable respective voltage to develop the respective latent image,which voltage can be supplied by a power supply (not shown). Developeris provided to toning shell 226 by a supply system (not shown) such as asupply roller, auger, or belt. Toner is transferred by electrostaticforces from development station 225 to photoreceptor 206. These forcescan include Coulombic forces between charged toner particles and thecharged electrostatic latent image, and Lorentz forces on the chargedtoner particles due to the electric field produced by the bias voltages.

In some embodiments, the development station 225 employs a two-componentdeveloper that includes toner particles and magnetic carrier particles.The exemplary development station 225 includes a magnetic core 227 tocause the magnetic carrier particles near toning shell 226 to form a“magnetic brush,” as known in the electrophotographic art. Magnetic core227 can be stationary or rotating, and can rotate with a speed anddirection the same as or different than the speed and direction oftoning shell 226. Magnetic core 227 can be cylindrical ornon-cylindrical, and can include a single magnet or a plurality ofmagnets or magnetic poles disposed around the circumference of magneticcore 227. Alternatively, magnetic core 227 can include an array ofsolenoids driven to provide a magnetic field of alternating direction.Magnetic core 227 preferably provides a magnetic field of varyingmagnitude and direction around the outer circumference of toning shell226. Development station 225 can also employ a mono-component developercomprising toner, either magnetic or non-magnetic, without separatemagnetic carrier particles.

Transfer subsystem 50 includes transfer backup member 113, andintermediate transfer member 112 for transferring the respective printimage from photoreceptor 206 of imaging member 111 through a firsttransfer nip 201 to surface 216 of intermediate transfer member 112, andthence to a receiver 42 which receives respective toned print images 38from each printing subsystem in superposition to form a composite imagethereon. The print image 38 is, for example, a separation of one color,such as cyan. Receiver 42 is transported by transport web 81. Transferto a receiver is effected by an electrical field provided to transferbackup member 113 by power source 240, which is controlled by LCU 99.Receiver 42 can be any object or surface onto which toner can betransferred from imaging member 111 by application of the electricfield. In this example, receiver 42 is shown prior to entry into asecond transfer nip 202, and receiver 42 a is shown subsequent totransfer of the print image 38 onto receiver 42 a.

In the illustrated embodiment, the toner image is transferred from thephotoreceptor 206 to the intermediate transfer member 112, and fromthere to the receiver 42. Registration of the separate toner images isachieved by registering the separate toner images on the receiver 42, asis done with the NexPress 2100. In some embodiments, a single transfermember is used to sequentially transfer toner images from each colorchannel to the receiver 42. In other embodiments, the separate tonerimages can be transferred in register directly from the photoreceptor206 in the respective printing subsystem 31, 32, 33, 34, 25 to thereceiver 42 without using a transfer member. Either transfer process issuitable when practicing this invention. An alternative method oftransferring toner images involves transferring the separate tonerimages, in register, to a transfer member and then transferring theregistered image to a receiver.

LCU 99 sends control signals to the charging subsystem 210, the exposuresubsystem 220, and the respective development station 225 of eachprinting subsystem 31, 32, 33, 34, 35 (FIG. 1), among other components.Each printing subsystem can also have its own respective controller (notshown) coupled to LCU 99.

Various finishing systems can be used to apply features such asprotection, glossing, or binding to the printed images. The finishingsystem scan be implemented as an integral components of the printer 100,or can include one or more separate machines through which the printedimages are fed after they are printed.

FIG. 3 shows a processing path that can be used to produce a printedimage 450 with a print engine 370 in accordance with embodiments of theinvention. A pre-processing system 305 is used to process a pagedescription file 300 to provide image data 350 that is in a form that isready to be printed by the print engine 370. In an exemplaryconfiguration, the pre-processing system 305 includes a digital frontend (DFE) 310 and an image processing module 330. The pre-processingsystem 305 can be a part of printer 100 (FIG. 1), or may be a separatesystem which is remote from the printer 100. The DFE 310 and an imageprocessing module 330 can each include one or more suitably-programmedcomputer or logic devices adapted to perform operations appropriate toprovide the image data 350.

The DFE 310 receives page description files 300 which define the pagesthat are to be printed. The page description files 300 can be in anyappropriate format (e.g., the well-known Postscript command file formator the PDF file format) that specifies the content of a page in terms oftext, graphics and image objects. The image objects are typicallyprovided by input devices such as scanners, digital cameras or computergenerated graphics systems. The page description file 300 can alsospecify invisible content such as specifications of texture, gloss orprotective coating patterns.

The DFE 310 rasterizes the page description file 300 into image bitmapsfor the print engine to print. The DFE 310 can include variousprocessors, such as a raster image processor (RIP) 315, a colortransform processor 320 and a compression processor 325. It can alsoinclude other processors not shown in FIG. 3, such as an imagepositioning processor or an image storage processor. In someembodiments, the DFE 310 enables a human operator to set up parameterssuch as layout, font, color, media type or post-finishing options.

The RIP 315 rasterizes the objects in the page description file 300 intoan image bitmap including an array of image pixels at an imageresolution that is appropriate for the print engine 370. For text orgraphics objects the RIP 315 will create the image bitmap based on theobject definitions. For image objects, the RIP 315 will resample theimage data to the desired image resolution.

The color transform processor 320 will transform the image data to thecolor space required by the print engine 370, providing colorseparations for each of the color channels (e.g., CMYK). For cases wherethe print engine 370 includes one or more additional colors (e.g., red,blue, green, gray or clear), the color transform processor 320 will alsoprovide color separations for each of the additional color channels. Theobjects defined in the page description file 300 can be in anyappropriate input color space such as RGB, CIELAB, PCS LAB or CMYK. Insome cases, different objects may be defined using different colorspaces. The color transform processor 320 applies an appropriate colortransform to convert the objects to the device-dependent color space ofthe print engine 370. Methods for creating such color transforms arewell-known in the color management art, and any such method can be usedin accordance with the present invention. Typically, the colortransforms are defined using color management profiles that includemulti-dimensional look-up tables. Input color profiles are used todefine a relationship between the input color space and a profileconnection space (PCS) defined for a color management system (e.g., thewell-known ICC PCS associated with the ICC color management system).Output color profiles define a relationship between the PCS and thedevice-dependent output color space for the printer 100. The colortransform processor 320 transforms the image data using the colormanagement profiles. Typically, the output of the color transformprocessor 320 will be a set of color separations including an array ofpixels for each of the color channels of the print engine 370 stored inmemory buffers.

The processing applied in digital front end 310 can also include otheroperations not shown in FIG. 3. For example, in some configurations, theDFE 310 can apply the halo correction process described incommonly-assigned U.S. Pat. No. 9,147,232 (Kuo) entitled “Reducing haloartifacts in electrophotographic printing systems,” which isincorporated herein by reference.

The image data provided by the digital front end 310 is sent to theimage processing module 330 for further processing. In order to reducethe time needed to transmit the image data, a compressor processor 325is typically used to compress the image data using an appropriatecompression algorithm. In some cases, different compression algorithmscan be applied to different portions of the image data. For example, alossy compression algorithm (e.g., the well-known JPEG algorithm) can beapplied to portions of the image data including image objects, and alossless compression algorithm can be applied to portions of the imagedata including binary text and graphics objects. The compressed imagevalues are then transmitted over a data link to the image processingmodule 330, where they are decompressed using a decompression processor335 which applies corresponding decompression algorithms to thecompressed image data.

A halftone processor 340 is used to apply a halftoning process to theimage data. The halftone processor 340 can apply any appropriatehalftoning process known in the art. Within the context of the presentdisclosure, halftoning processes are applied to a continuous-tone imageto provide an image having a halftone dot structure appropriate forprinting using the printer module 435. The output of the halftoning canbe a binary image or a multi-level image. In an exemplary configuration,the halftone processor 340 applies the halftoning process described incommonly assigned U.S. Pat. No. 7,830,569 (Tai et al.), entitled“Multilevel halftone screen and sets thereof,” which is incorporatedherein by reference. For this halftoning process, a three-dimensionalhalftone screen is provided that includes a plurality of planes, eachcorresponding to one or more intensity levels of the input image data.Each plane defines a pattern of output exposure intensity valuescorresponding to the desired halftone pattern. The halftoned pixelvalues are multi-level values at the bit depth appropriate for the printengine 370.

The image enhancement processor 345 can apply a variety of imageprocessing operations. For example, an image enhancement processor 345can be used to apply various image enhancement operations. In someconfigurations, the image enhancement processor 345 can apply analgorithm that modifies the halftone process in edge regions of theimage (see U.S. Pat. No. 7,079,281, entitled “Edge enhancement processorand method with adjustable threshold setting” and U.S. Pat. No.7,079,287 entitled “Edge enhancement of gray level images” (both to Nget al.), and both of which are incorporated herein by reference).

The pre-processing system 305 provides the image data 350 to the printengine 370, where it is printed to provide the printed image 450. Thepre-processing system 305 can also provide various signals to the printengine 370 to control the timing at which the image data 350 is printedby the print engine 370. For example, the pre-processing system 305 cansignal the print engine 370 to start printing when a sufficient numberof lines of image data 350 have been processed and buffered to ensurethat the pre-processing system 305 will be capable of keeping up withthe rate at which the print engine 370 can print the image data 350.

A data interface 405 in the print engine 370 receives the data from thepre-processing system 305. The data interface 405 can use any type ofcommunication protocol known in the art, such as standard Ethernetnetwork connections. A printer module controller 430 controls theprinter module 435 in accordance with the received image data 350. In anexemplary configuration, the printer module 435 can be the printer 100of FIG. 1, which includes a plurality of individual electrophotographicprinting subsystems 31, 32, 33, 34, 35 for each of the color channels.For example, the printer module controller 430 can provide appropriatecontrol signals to activate light sources in the exposure subsystem 220(FIG. 2) to exposure the photoreceptor 206 with an exposure pattern. Insome configurations, the printer module controller 430 can apply variousimage enhancement operations to the image data. For example, analgorithm can be applied to compensate for various sources ofnon-uniformity in the printer 100 (e.g., streaks formed in the chargingsubsystem 210, the exposure subsystem 220, the development station 225or the fuser module 60). One such compensation algorithm is described incommonly-assigned U.S. Pat. No. 8,824,907 (Kuo et al.), entitled“Electrophotographic printing with column-dependent tonescaleadjustment,” which is incorporated herein by reference.

In some cases, the printing system can also include an image capturesystem 440. The image capture system can be used for purposes such assystem calibration. The image capture system 440 can use any appropriateimage capture technology such as a digital scanner system, or a digitalcamera system. The image capture system 440 can be integrated into theprinting system, or can be a separate system which is in communicationwith the printing system.

In the configuration of FIG. 3, the pre-processing system 305 is tightlycoupled to the print engine 370 in that it supplies image data 350 in astate which is matched to the printer resolution and the halftoningstate required for the printer module 435. In other configurations, theprint engine can be designed to be adaptive to the characteristics ofdifferent pre-processing systems 305 as is described incommonly-assigned, co-pending U.S. patent application Ser. No.15/135,607 (now U.S. Publication No. 2017/0308774) to Kuo et al.,entitled “Print engine with adaptive processing,” which is incorporatedherein by reference.

Aspects of the present invention will now be described with reference toFIG. 4, which shows a flow chart of processing operations that can beused to apply various calibration and artifact correction processes inaccordance with exemplary embodiments. Some of the operations can beapplied in data processing electronics 570 before passing the image datato the printer module 435 (e.g., in the printer module controller 430(FIG. 3)), while other operations can be applied in printheadelectronics 580 associated with the exposure subsystem 220 (FIG. 2) ofthe printer module 435.

The input to the flow chart is a pixel code value 500 for an image pixelin an array of image data to be printed by one of theelectrophotographic printing subsystems 31, 32, 33, 34, 35 in theprinter 100. In an exemplary embodiment, the pixel code value 500 can bea pixel of the image data 350 that is input to the print engine 370 (seeFIG. 3). Typically, the pixel code value 500 will be an 8-bit numberbetween 0-255.

An apply calibration LUT step 510 is used to apply a calibrationlook-up-table (LUT) 505 to the pixel code value 500. Typically, theoutput of the calibration LUT will be an exposure value EV which islinear with the exposure level to be provided by the printhead. In anexemplary arrangement, the exposure value EV is represented by a 12-bitinteger in the range 0-4095. The exposure value EV corresponds to theexposure that should be provided to the photoreceptor 206 (FIG. 2) bythe exposure subsystem 202 such that the printer 100 (FIG. 1) producesan aim density value appropriate for the pixel code value 500.

An apply gain corrections step 520 is used to apply gain correctionvalues 515 on a pixel-by-pixel basis to compensate for various sourcesof non-uniformity in the printer 100 (e.g., streaks formed in thecharging subsystem 210, the exposure subsystem 220, the developmentstation 225 or the fuser module 60). In an exemplary embodiment, theapply gain corrections step 520 applies the compensation algorithmdescribed in the aforementioned U.S. Pat. No. 8,824,907. This methodinvolves determining two gain correction values 515 (i.e., G1 and G2)for each light source in the linear printhead. The output of the applygain corrections step 520 is a modified exposure value EV′.

While the exposure value EV is a 12-bit number in an exemplaryconfiguration, only 256 of the different code values will be used sincethe pixel code value 500 is an 8-bit number. The apply gain correctionsstep 520 will modify the exposure value EV for each light source in adifferent manner in accordance with the associated gain correctionvalues 515. As a result, the modified exposure values EV′ will generallyutilize many more of the available 12-bit code values. The exact set ofcode values that are used will depend on the gain correction values 515that are necessary to correct for the streak artifacts.

The interface to the printhead is typically an 8-bit number. As aresult, it is necessary to use an apply quantization step 530 todetermine a quantized exposure value 540 by applying an appropriatequantization LUT 525. To minimize quantization errors, a vectorquantization process can be used to select the ranges of exposure valueswhich are mapped to each of the quantized exposure values 540. Vectorquantization processes are well-known in the art and any appropriateprocess can be used in accordance with the present invention. An exampleof a quantization LUT 525 is shown in FIG. 5. The quantization LUT 525defines a set of bins B_(i) that correspond to the range of modifiedexposure values that are mapped to the i^(th) quantized exposure value.An aim exposure value E_(a,i) can also be defined for each binspecifying an aim exposure value that is representative of the i^(th)quantized exposure value. The set of aim exposure values define an aimexposure function 605, which can be represented as a vector E_(a):E _(a) =[E _(a,0) ,E _(a,1) , . . . E _(a,i) , . . . E _(a,255)]  (1)An exemplary aim exposure function 605 is illustrated in FIG. 6.

Over time, it has been found that the characteristics of the streakartifacts can change. It is therefore desirable to perform a calibrationprocess to determine the light-source-dependent gain correction values515 on a periodic or as needed basis. For example, the calibrationprocess can be performed at the beginning of each day, or can beinitiated if an operator observes the presence of streak artifacts.Since the optimal quantization LUT 525 will be a function of the gaincorrection values 515, it is generally desirable to determine an updatedquantization LUT 525 at the same time. In a preferred embodiment, adetermine gain corrections process 590 is performed as part of thecalibration process to determine the gain correction values 515 for eachlight source, the quantization LUT 525 and the corresponding aimexposure function 605.

The quantized exposure values 540 are passed to the printhead where theyare used to control the exposure provided by the corresponding lightsources. In an exemplary embodiment, a control light source exposuretime step 550 controls the exposure by activating each light source inthe printhead for an exposure time needed to provide the aim exposurevalue E_(a,i) corresponding to the associated quantized exposure value540.

In some embodiments, the printhead has an associated master clock whichprovides a master clock signal 660 as shown in FIG. 7. For example, themaster clock can run at 80 MHz. An exposure clock signal 670 is thenformed having a stream of pulses formed by counting out a correspondingnumber of pulses in the master clock signal 660. The exposure can thenbe controlled by activating the light source at time t=0, and thendeactivating the light source after counting a number of exposure clocksignal pulses corresponding to the quantized exposure value 540. Thetime (t) for the i^(th) pulse is given by pulse time S_(i), The set ofpulse times for each of the quantized exposure values together define apulse timing function 610 (S):S=[S ₀ ,S ₁ , . . . S _(i) , . . . S ₂₅₅]  (2)In an exemplary configuration, the pulse times S_(i) are represented interms of the number of master clock pulses. FIG. 7 illustrates a lightsource activation function 680 corresponding to a quantized exposurevalue 540 of EQ=5 where the light source is activated at time t=0 anddeactivated at time S₅ when the falling edge of the 5^(th) exposureclock signal pulse is detected.

In the simplest case, the power (i.e., the light output) provided by thelight sources is constant during the time that the light source isactivated so that the exposure will simply be proportional to theexposure time. However, it has been found that the power provided by thelight source typically varies with time (for example, see the exemplarylight output function 630 in FIG. 9B). To further complicate matters,the time dependency varies as a function of the pulse times which makeup the exposure clock signal 670. For example, for some common driverchips used in LED printhead it has been found that when the pulses inthe exposure clock signal 670 are closer together the light output istypically lower than when the pulses in the exposure clock signal 670are farther apart.

A determine pulse timing function process 600 is used to determine thepulse timing function 610 that will deliver the specified aim exposurefunction 605. To determine the pulse timing function 610 it is necessaryto know the shape of the light output function 630 in order to be ableto compute the exposure provided to a particular exposure time. But, ashas been discussed, the shape of the light output function 630 dependson the pulse timing function 610. Consequently, it is not possible todetermine the pulse timing function 610 using a straightforward process.

FIG. 8 illustrates an iterative process that has been developed for useby the determine pulse timing function process 600 in accordance with anexemplary embodiment. A determine light output function step 620 is usedto determine an initial light output function 630 based on an initialpulse timing function 615 (S⁰). The initial pulse timing function 615can be provided in a variety of ways. In some embodiments, it can be apreviously determined pulse timing function which was determined for asimilar aim exposure function 605. In other embodiments the initialpulse timing function 615 can be determined based on the assumption thatthe light output function 630 is constant with time.

The determine light output function step 620 can determine the lightoutput function 630 using any appropriate means. In one exemplaryconfiguration, one or more light sources in the printhead can becontrolled using the initial pulse timing function 615 and the lightoutput function 630 can be measured using a light detector whichmeasures light output of the one or more light sources as a function ofexposure time. In a preferred configuration, the determine light outputfunction step 620 determines the light output function 630 using a lightoutput model 625 which predicts the light output as a function ofexposure time given a pulse timing function.

It has been found that the following functional form for the lightoutput model 625 produces good predictions for the normalized lightoutput as a function of exposure time for a common type driver chips forLED printheads (e.g., the model LC46611C drier chip available from ONSemiconductor):

$\begin{matrix}{{P(t)} = {{1 - \frac{\alpha}{\Delta\; t_{i}}} = {1 - \frac{\alpha}{\left( {S_{i + 1} - S_{i}} \right)}}}} & (3)\end{matrix}$where S_(i) is the i^(th) pulse time, Δt_(i)=(S_(i+1)−S_(i)) is the timedifference between two successive exposure clock signal pulses at timet, and α is a constant which can be experimentally determined for thedriver chip and operating conditions. A typical value of α would be onthe order of 0.01-0.02 msec.

Next, an update pulse timing function step 635 is performed to determinean updated pulse timing function 640 that would provide the exposurevalues given by the aim exposure function 605 given the determined lightoutput function 630. The updated pulse time S_(i) ^(j) for the i^(th)quantized exposure value 540 and the j^(th) iteration can be determinedby computing the updated pulse time that satisfies the followingequation:

$\begin{matrix}{{\hat{E}}_{a,i} = {\frac{E_{a,i}}{E_{a,255}} = \frac{\int_{0}^{S_{i}^{j}}{{P^{({j - 1})}(t)}{dt}}}{\int_{0}^{S_{255}^{j}}{{P^{({j - 1})}(t)}{dt}}}}} & (4)\end{matrix}$where P^((j-1))(t) is the light output function 630 determined for theprevious iteration, E_(a,i) is the aim exposure value for the i^(th)quantized exposure value 540, and Ê_(a,i) is the correspondingnormalized aim exposure value. (Note that this approach determines theupdated pulse timing function 640 that would provide the exposure valueshaving the same normalized shape the aim exposure function 605. Theabsolute exposure value can be matched by adjusting the overall currentprovided to the light sources.) The updated pulse times S_(i) ^(j) thatsatisfy this equation can be determined using well-known numericalintegration techniques. The updated pulse timing function 640 (S^(j))for the j^(th) iteration corresponds to the vector of individual pulsetimes:

$\begin{matrix}{S^{j} = \left\lbrack {S_{0}^{j},S_{1}^{j},{\ldots\mspace{14mu} S_{i}^{j}},{\ldots\mspace{14mu} S_{255}^{j}}} \right\rbrack} & (5)\end{matrix}$

A done test 645 is used to determine whether or not a predefinediteration termination criterion is satisfied. In one exemplaryembodiment, the updated pulse timing function 640 (S^(j)) is compared tothe pulse timing function for the previous iteration (S^((j-1))) todetermine whether the results have converged. For example, the iterationtermination criterion can be evaluated by determining the magnitude of avector difference between the two pulse timing functions and comparingit to a predefined threshold ε_(s):

$\begin{matrix}{{{S^{j} - S^{({j - 1})}}} < ɛ_{s}} & (6)\end{matrix}$In other variations, rather than determining the magnitude of the vectordifference, the maximum difference can be determined for the elements ofthe vector difference. In this case, if there was a significantdifference in the pulse timing for one quantized exposure value, theiterative process would continue even if the total difference was small.

In another embodiment, the iteration termination criterion can includecomputing an normalized actual exposure function for the currentiteration (Ê^(j)) and comparing it to the normalized aim exposurefunction 605 (Ê_(a)). For example, the iteration termination criterioncan be evaluated by determining the magnitude of a vector differencebetween the exposure functions and comparing it to a predefinedthreshold ε_(e):

$\begin{matrix}{{{{\hat{E}}^{j} - {\hat{E}}_{a}}} < ɛ_{e}} & (7)\end{matrix}$where the normalized actual exposure function for the current iteration(Ê^(j)) is determined by integrating the light output function P^(j) (t)for the current iteration using the corresponding pulse times in theupdated pulse timing function (S^(j)):

$\begin{matrix}{{\hat{E}}_{i}^{j} = \frac{\int_{0}^{S_{i}^{j}}{{P^{j}(t)}{dt}}}{\int_{0}^{S_{255}^{j}}{{P^{j}(t)}{dt}}}} & (8)\end{matrix}$

The pulse timing function 610 that provides the specified aim exposurefunction 605 can be a function of the printer configuration. Forexample, some printers can be configured to print at a variety ofin-track spatial resolutions (e.g., 600 dpi or 1200 dpi). If the overallprint speed is maintained to be the same, this means that the 1200 dpipixels must be printed in half the time as the 600 dpi pixels. As aresult, the associated pulse times will nominally be about half as longas well. This will typically have a significant impact on the shape ofthe light output function 630, and will therefore require that the pulsetiming function 610 be reoptimized accordingly. Therefore, in suchcases, it can be necessary to apply the method of FIG. 8 to determine anappropriate pulse timing function 610 for each of the relevant printerconfigurations. Each of the resulting pulse timing functions 610 can bestored and used when the printer is used in the correspondingconfiguration.

If the done test 645 determines that the iteration termination criterionhas been satisfied, a store final pulse timing function step 650 is usedto store the results of the final iteration as the pulse timing function610 (S) in a processor accessible memory for use in controlling theprinthead to print image data. Otherwise, another iteration is performedby applying the determine light output function step 620 and the updatepulse timing functions step 635 again. It has been found that theprocess typically converges in 10-200 iterations.

FIG. 9A shows an example of an initial pulse timing function 615. Thecorresponding initial light output function 630 is shown in FIG. 9B. Thedetermine pulse timing function process 600 of FIG. 8 was applied usingthis initial pulse timing function 615 to determine the updated pulsetiming function 610 shown in FIG. 9A that will provide the aim exposurefunction 605 of FIG. 6. The corresponding optimized light outputfunction 632 is shown in FIG. 9B. It can be seen that the optimizedlight output function 632 is quite different than the initial lightoutput function 630. This demonstrates the dependency of the lightoutput function on the shape of the pulse timing function.

Returning to a discussion of FIG. 4, the pulse timing function 610determined by the determined pulse timing function process 600 is usedby a control light source exposure time step 550, which is applied inthe printhead electronics 580 to control how long each of the individuallight sources in the printhead is activated in response to thecorresponding quantized exposure value 540.

In an exemplary embodiment, the same pulse timing function 610 is usedfor all of the light sources in the linear printhead. However, therewill generally be differences between the light output of the differentlight sources when they are operated at the same current. This canresult in various artifacts in the printed images such as streaks. Tocompensate for these artifacts, the current supplied to each lightsource can be adjusted using a control light source current step 550 toequalize the light output of the light sources. A calibration operationincluding a determine current control parameters process 700 can beperformed to determine a set of current control parameters 710 that areused by the control light source current step 560 to control the currentfor each light source.

In some embodiments, the determine current control parameters process700 can determine the current control parameters 710 by placing theprinthead into a test fixture that includes a light sensor and measuringthe light output for each light source. In this way, the currentsupplied to each light source can be adjusted until the light outputfrom each light source is equalized to within a predefined tolerance.

In an exemplary embodiment, a plurality of driver chips is used tocontrol the light sources in the printhead, wherein each driver chipcontrols an associated set of light sources. For example, a printhead inan exemplary printing system includes a linear array of 17,280 lightsources that are controlled by 90 driver chips, where each driver chipcontrols 17,280/90=192 light sources. In this case, the printhead isdivided into 45 segments along its length. Within each segment onedriver chip controls the odd-numbered light sources, and a second driverchip controls the even-numbered light sources.

In an exemplary configuration, the current control parameters 710include a global current control value (V_(REF)), a set ofchip-dependent current control values (C_(REF)), and a set ofsource-dependent current control values (D_(REF)). The global currentcontrol value (V_(REF)) is a parameter which sets an overall currentlevel I_(G) which is supplied to all of the light sources in theprinthead.

The chip-dependent current control values (C_(REF)) can be representedby an array of control values (one for each driver chip) that are usedto independently adjust the current provided by each of the driverchips:C _(REF) =[C ₁ ,C ₂ , . . . C _(m) , . . . C _(M)]  (9)where M is the number of driver chips, and C_(m) is the chip-dependentcurrent control value for the m^(th) driver chip. In an exemplaryconfiguration, each C_(m) value is a 4-bit integer ranging from 0-15that specifies a gain adjustment in 3% increments. In this case, thechip-dependent gain adjustment can be expressed asG_(c,m)=0.03×(C_(m)−7).

The source-dependent current control values (D_(REF)) can be representedby an array of control values (one for each light source) that are usedto independently adjust the current provided by each of the lightsources:D _(REF) =[D ₁ ,D ₂ , . . . D _(n) , . . . D _(N)]  (10)where N is the number of light sources, and D_(n) is thesource-dependent current control value for the n^(th) light source. Inan exemplary configuration, each D_(n) value is a 6-bit integer rangingfrom 0-63 that specifies a gain adjustment in 1% increments. In thiscase, the source-dependent gain adjustment can be expressed asG_(d,n)=0.01×(D_(n)−31).

The current supplied to each light source will be the global current asmodified by the chip-dependent gain adjustment and the source-dependentgain adjustment. In equation form, the current supplied to the n^(th)light source that is controlled by the m^(th) driver chip is given by:I _(n) =I _(G)(1+G _(c,m) +G _(d,n))=I _(G)(1+0.03×(C _(m)−7)+0.01×(D_(n)−31))  (11)

FIG. 10 illustrate a flowchart of an exemplary embodiment of a determinecurrent control parameters process 700 which determines the currentcontrol parameters 710 based on the analysis of a printed test target.In this process, the printhead is configured to use a set of initialcurrent control parameters 715. The initial current control parameters715 can be obtained in a variety of ways. For example, they can be a setof current control parameters determined using a test fixture thatincludes a light sensor and measures the light output for each lightsource as discussed earlier. Alternately, they can be a set of currentcontrol parameters determined using a previous calibration process.

A print test target step 725 is used to print test target image data 720for a test target 760 including one or more uniform patches. FIG. 11illustrates an exemplary test target 760 that can be used in anexemplary embodiment. The test target 760 includes a set of uniformpatches 800, which span the width of the printhead in the cross-trackdirection 810. Each uniform patch 800 is positioned at a differentin-track position in the in-track direction 812. Each of the uniformpatches 800 has a different density level ranging from a lighted uniformpatch 802 to a darkest uniform patch 804. The test target 760 alsoincludes a set of alignment marks 806 having known positions relative tothe printhead that can be used to determine the alignment of the printedtest target to the printhead.

Generally, continuous tone digital image data for the test target 760 isprocessed through a halftoning process before it is printed to providehalftoned image data. In an exemplary embodiment, the halftoning processis a stochastic halftoning process. The use of a stochastic halftoningprocess is advantageous because its characteristics are more isotropicand less prone to moiré artifacts during the image capture process. Thehalftoned image data is then printed using the process of FIG. 4.Preferably, during the process of determining the current controlparameters 710, the gain correction values 515 are all set to unityvalues so that no gain corrections are applied by the apply gaincorrections step 520.

The printed test target 730 produced by the print test target step 725is next digitized using a scan test target step 735. The scan testtarget image step 735 uses a digital image capture system 440 (FIG. 3)to provide a captured image 749 of the printed test target 730. In apreferred embodiment, the digital image capture system 440 is a digitalcamera system or an optical scanner system that is integrated into thedigital printing system. In some configurations the digital imagecapture system 440 is used to automatically capture the image of theprinted test target 730 as it travels through the digital printingsystem.

An analyze captured image step 745 is next used to analyze the capturedimage 740 to determine estimated light-source-dependent exposure errors750. FIG. 12 shows a flowchart for an exemplary process that can be usedto perform the analyze captured image step 745. First, an align imagestep 900 is used to detect the locations of the alignment marks 806(FIG. 11) and remove any skew from the captured image 740. A determinelight source positions step 905 determines a cross-track position ofeach light source within the image based on the detected locations ofthe alignment marks 806.

A determine light-source-dependent code values step 910 is then used todetermine an average code value within each uniform patch 800 for eachlight source. This is done by averaging the code values in a verticalcolumn within the uniform patch at the determined cross-track positionfor the light source. FIG. 13 shows a graph 920 illustrating a sampleset of curves showing the scanner code value as a function of lightsource for a set of six uniform patches. (Note that a set of lightsources on either end of the head were outside the active printing areaof the printing system so that the number of light sources in the graph920 is less than the total number of light sources in the printhead.)

Returning to a discussion of FIG. 12, a determine light-source-dependentexposure errors step 915 is then used to determine correspondingestimated light-source-dependent exposure errors 750. In an exemplaryembodiment, the digitized scanner code values are mapped to exposurevalues by applying a calibration curve 930 such as that shown in FIG.14. The calibration curve 930 can be determined by printing patcheshaving known exposures and measuring the resulting code values in ascanned image. Note that the “exposure” values in FIG. 14 and subsequentplots are the exposure times that the light source is activated in unitsof microseconds. These values will be proportional to the actualexposure, which can be determined by multiplying these values by thepower of the light source (which is about 180 picowatts).

To evaluate the exposure errors, the measured exposure values vs. lightsource functions can be smoothed (e.g., by fitting a spline function) todetermine a set of smoothed exposure values. The difference between thesmoothed and unsmoothed functions will be an estimate of the exposureerrors for each of the light sources. FIG. 15 shows a graph 940 showingthe estimated exposure error as a function of light source for one ofthe uniform patches 800 (FIG. 11).

Returning to a discussion of FIG. 10, a determine updated currentcontrol parameters step 755 is next used to determine the updatedcurrent control parameters 710. In an exemplary embodiment, an exposuregain error is determined for each of the light sources by combining theestimated exposure errors for each of the uniform patches 800. FIG. 16is a graph 950 showing the estimated exposure error determined from thesix uniform patches 800 (FIG. 11) for two of the light sources. A linearfunction can be fit to the points for each light source to provide anestimated gain error. In a preferred embodiment, the linear function isconstrained to go through the origin, and the slope of the resultinglinear function is therefore an estimate of the exposure gain error. Apositive slope indicates that the light source is providing too muchexposure and a negative slope is an indication that the light source isproviding too little exposure.

FIG. 17 shows a graph 960 illustrating an exemplary set of gaincorrections determined for each of the light sources. (In this plot, thex-axis has been scaled to the number of control chips across theprinthead.) These gain corrections can then be combined with the gainvalues associated with the initial current control parameters 715 (FIG.10) to determine an updated set of gain adjustment values. The updatedgain adjustment values are then used to determine a corresponding set ofcurrent control parameters 710.

In an exemplary embodiment, the global current control value (V_(REF))is not adjusted during this process, so the same value is used as in theinitial current control parameters. Rather, the value of the globalcurrent control value (V_(REF)) is set to produce the desired maximumexposure level at a quantized exposure value 540 of EQ=255. To determinethe set of chip-dependent current control values (C_(REF)) for theupdated current control parameters 710, the gain adjustment valuesassociated with each of the control chips are averaged and quantizedinto bins associated with the available chip-dependent current controlvalues (C_(m)). The associated chip-dependent gain adjustment iscalculated for each control chip (e.g., using the equationG_(c,m)=0.03×(C_(m)−7)) and is subtracted from the gain adjustmentvalues to determine residual gain adjustment values. The residual gainadjustment values for each light source are quantized into binsassociated with the available source-dependent current control value(D_(n)). The chip-dependent current control values (C_(m)) are used toform the vector of chip-dependent current control values (C_(REF)) andthe source-dependent current control values (D_(n)) are used to form thesource-dependent current control values (D_(REF)) for the updatedcurrent control parameters 710. A plot of the resulting chip-dependentcurrent control values is shown in graph 962, and a plot of theresulting source-dependent current control values is shown in graph 964.

Once the updated current control parameters 710 are determined, they arestored in a processor-accessible memory for use in printing subsequentdigital image data. In some embodiments, the determine current controlparameters process 700 of FIG. 10 can be performed iteratively tofurther refine the gain corrections, where the updated current controlparameters 710 are used as the initial current control parameters forthe next iteration. For example, the determine current controlparameters process 700 can be repeated until the determinedlight-source-dependent exposure errors 750 are all less than apredefined threshold value.

Returning to a discussion of FIG. 4, in an exemplary embodiment, thedetermine current control parameters process 700 is performed in thefactory to determine a set of current control parameters 710 that arestored in the printing system when it is shipped to a customer.Typically, the determine gain corrections process 590 will be used inthe field to correct for any streak artifacts that arise in the printedimages (e.g., due to degradation of the printhead or other componentssuch as the charging subsystem 210 or the development subsystem 225).However, the determine current control parameters process 700 can alsobe performed in the field on an as-needed basis. For example, thedetermine current control parameters process 700 can be performed when anew printhead is installed or when a service technician observes thatperformance degradations have occurred. When the determine currentcontrol parameters process 700 is performed, the gain correction values515 and the quantization LUT 525 are typically set to nominal values.After the updated current control parameters 710 are determined, thedetermine gain corrections process 590 can be performed to correct forany residual errors that may remain.

As was discussed earlier with respect to FIG. 8, it has been found thatdifferent pulse timing functions 610 may be needed to provide a definedaim exposure function 605 depending on the printer configuration. Inparticular, different pulse timing functions 610 will typically beneeded for different print modes having different line print times(i.e., the time it takes for the printhead to print a line of imagedata). The line print time will define the maximum pulse time that canbe used for the pulse timing function 610, which will in turn have asignificant effect on the light output function 630. The aspects of theprint mode that will have a direct impact on the line print time will bethe in-track printer resolution (i.e., the number of lines/inch that areprinted that are printed in the in-track direction, and the print speed(i.e., the number of pages/minute that are printed). For example,doubling the in-track printer resolution or doubling the print speedwill have the effect of reducing the line print time by a factor of 2×.

In an exemplary embodiment, the printing system is adapted to print at aset of different print modes having the following characteristics:

TABLE 1 Exemplary Print Modes In-Track Printer Print Speed Line PrintPrint Resolution (pages/ Time Mode (lines/inch) minute) (μsec) 1 1200 83 21.1 2 1200 100 17.5 3  600  83 42.2 4  600 100 35.0 5  600 120 29.2Each of these five print modes has a different line print time, and as aresult requires a different pulse timing function 610 in order toprovide a defined aim exposure function 605.

In some embodiments, a user interface can be provided (e.g., in apre-processing module 305) that enables a user to select a differentprint mode on a job-by-job basis. Therefore, in a preferred embodiment,a mechanism is provided to select the appropriate pulse timing functionto be used with each print job. For example, FIG. 18 shows an exemplaryuser interface 970 having user selectable options for specifying aspectsof a print mode. In this example, the user selections for specifying theprint mode include a resolution selection 972 for selecting an in-trackprinter resolution and a print speed selection 974 for selecting a printspeed. While the resolution selection 972 and the print speed selection974 are shown with numerical choices, in other embodiments text labelscould be used. For example, the 1200 lines/inch printer resolution couldbe labeled “MaxHD” and the 600 lines/inch printer resolution could belabeled “Classic.”

In an exemplary embodiment, only certain combinations of the printerresolution and the print speed may be allowable. For example, if a 1200lines/inch printer resolution is selected, the print speed choices maybe limited to 82 pages/minute or 100 pages/minute so that the 120pages/minute selection is dimmed out. The user interface 970 can alsoinclude other selections for controlling other attributes of the printjob (e.g., number of copies to print, pages to print, type of halftoningto be applied, etc.).

FIG. 19 shows a processing path including a print engine 400 that isadapted to produce printed images from image data 350 using a pluralityof print modes. This processing path represents an extension of thatdescribed in the aforementioned U.S. patent application Ser. No.15/135,607 (now U.S. Publication No. 2017/0308774) to Kuo et al. In thisconfiguration, the pre-processing system 305 provides image data 350 aswell as associated metadata 360. In a preferred embodiment, the metadata360 includes print mode metadata that provides an indication of theprint mode that is to be used to print the image data 350. In anexemplary configuration, the print mode metadata can be an integerspecifying a print mode from a predefined set of print modes such asthose shown in Table 1. In other configurations, the print mode metadatacan include various parameters specifying various attributes of theprint mode, such as a printer resolution parameter and a print speedparameter that are specified using user interface 970 (FIG. 18). Themetadata 360 can also include other parameters such as image resolutionmetadata and halftoning state metadata.

The print engine 400 receives the image data 350 and the metadata 360using an appropriate data interface 405 (e.g., an Ethernet interface).The print engine includes a metadata interpreter 410 that analyzes themetadata 360 to provide appropriate control signals 415 that are used tovarious aspects of the print engine 400. In an exemplary configuration,the control signals include resolution modification control signals thatare used to control a resolution modification processor 420 and halftonealgorithm control signals that are used to control a halftone processor425 as described in the aforementioned U.S. patent application Ser. No.15/135,607 (now U.S. Publication No. 2017/0308774) to Kuo et al. Theresolution modification processor 420 and the halftone processor 425 areused to process the image data 350 to provide processed image data 428,which is in an appropriate state to be printed by the printer module435. A printer module controller 430 then controls the printer module435 to print the processed image data 428 to produce the printed image450.

In a preferred embodiment, the control signals 415 include a pulsetiming function selection parameter which is used to select a pulsetiming function 610 (FIG. 8). The metadata interpreter 410 determinesthe pulse timing function selection parameter responsive to metadata 360that specifies the print mode to be used to print the image data 350. Inan exemplary configuration, the print mode metadata includes an in-trackprinter resolution parameter that specifies an in-track printerresolution (e.g., 600 lines/inch or 1200 lines/inch) and a print speedparameter that specifies a print speed (e.g., 83 pages/minute, 100pages/minute or 120 pages/minute). As illustrated in Table 1, a set ofprint modes can be defined corresponding to allowable combinations ofthese parameters, each print mode having an associated line print time.In addition to selecting a pulse timing function 610, the controlsignals 415 determined from the print mode metadata can also includeparameters for controlling other aspects of the printer module 435. Forexample, the control signals 415 can be used to select a set of currentcontrol parameters 710 (FIG. 4) appropriate for the selected print mode,and to adjust the speed of various motors to control the print speed.

The pulse timing functions 610 for each of the print modes arepreferably pre-determined using the method of FIG. 8 for the line printtimes associated with each of the supported print modes and stored in aprocessor-accessible digital memory 460. FIG. 20 shows an exemplary setof pulse timing functions 610 corresponding to the print modes inTable 1. The pulse timing function selection parameter included in thecontrol signals 415 is used to select the appropriate pulse timingfunction 610 for the selected print mode, which is then used by theprinter module controller 430 to control the printhead in the printermodule 435.

The invention has been described in detail with particular reference tocertain preferred embodiments thereof, but it will be understood thatvariations, combinations, and modifications can be effected by a personof ordinary skill in the art within the spirit and scope of theinvention.

PARTS LIST  31 printing module  32 printing module  33 printing module 34 printing module  35 printing module  38 print image  39 fused image 40 supply unit  42 receiver  42a receiver  42b receiver  50 transfersubsystem  60 fuser module  62 fusing roller  64 pressure roller  66fusing nip  68 release fluid application substation  69 output tray  70finisher  81 transport web  86 cleaning station  99 logic and controlunit (LCU) 100 printer 111 imaging member 112 intermediate transfermember 113 transfer backup member 201 first transfer nip 202 secondtransfer nip 206 photoreceptor 210 charging subsystem 211 meter 212meter 213 grid 216 surface 220 exposure subsystem 225 developmentstation 226 toning shell 227 magnetic core 240 power source 300 pagedescription file 305 pre-processing system 310 digital front end (DFE)315 raster image processor (RIP) 320 color transform processor 325compression processor 330 image processing module 335 decompressionprocessor 340 halftone processor 345 image enhancement processor 350image data 360 metadata 400 print engine 405 data interface 410 metadatainterpreter 415 control signals 420 resolution modification processor425 halftone processor 428 processed image data 430 printer modulecontroller 435 printer module 440 image capture system 450 printed image460 digital memory 500 pixel code value 505 calibration LUT 510 applycalibration LUT step 515 gain correction values 520 apply gaincorrections step 525 quantization LUT 530 quantization step 540quantized exposure value 550 control light source exposure time step 560control light source current step 570 data processing electronics 580printhead electronics 590 determine gain corrections process 600determine pulse timing function process 605 aim exposure function 610pulse timing function 615 initial pulse timing function 620 determinelight output function step 625 light output model 630 light outputfunction 632 optimized light output function 635 update pulse timingfunction step 640 updated pulse timing function 645 done test 650 storefinal pulse timing function 660 master clock signal 670 exposure clocksignal 680 light source activation function 700 determine currentcontrol parameters process 710 current control parameters 715 initialcurrent control parameters 720 test target image data 725 print testtarget step 730 printed test target 735 scan test target step 740captured image 745 analyze captured image step 750light-source-dependent exposure errors 755 determine updated currentcontrol parameters step 760 test target 800 uniform patch 802 lightestuniform patch 804 darkest uniform patch 806 alignment mark 810cross-track direction 812 in-track direction 900 align image step 905determine light source positions step 910 determinelight-source-dependent code values step 915 determinelight-source-dependent exposure errors step 920 graph 930 calibrationcurve 940 graph 950 graph 960 graph 962 graph 964 graph 970 userinterface 972 resolution selection 974 print speed selection

The invention claimed is:
 1. A method for controlling a printhead in adigital printing system, the printhead including an array of lightsources for exposing a photosensitive medium, comprising: a) providingan aim exposure function which gives an aim exposure to be provided bythe light sources as a function of an integer pulse count; b) providingan initial pulse timing function which defines an exposure time as afunction of pulse count; c) determining a light output function for thelight sources responsive to the pulse timing function, wherein the lightoutput function gives a light output of the light sources as a functionof exposure time; d) updating the pulse timing function responsive tothe light output function and the aim exposure function; e) repeatingsteps c)-d) until a predefined iteration termination criterion issatisfied; and f) using the pulse timing function to control theprinthead, wherein each light source is activated for a pulse countcorresponding to a pixel code value of an associated image pixel.
 2. Themethod of claim 1, wherein the iteration termination criterion is that adifference between the updated pulse timing function and the pulsetiming function for a preceding iteration is no more than a predefinedthreshold.
 3. The method of claim 2, wherein the difference between theupdated pulse timing function and the pulse timing function for thepreceding iteration is a magnitude of a vector difference between avector representing the updated pulse timing function and a vectorrepresenting the pulse timing function for the preceding iteration. 4.The method of claim 1, wherein the iteration termination criterion isthat a difference between the aim exposure function and an actualexposure function is no more than a predefined threshold, wherein theactual exposure function is the actual exposure provided by the lightsources as a function of pulse count and is determined responsive to theupdated pulse timing function and the updated light output function. 5.The method of claim 4, wherein the difference between the aim exposurefunction and an actual exposure function is a magnitude of a vectordifference between a vector representing the aim exposure function and avector representing the actual exposure function.
 6. The method of claim1, wherein the light output function is determined using a predefinedparametric light output model.
 7. The method of claim 6, wherein theparametric light output model has the form:${P(t)} = {1 - \frac{\alpha}{\Delta\; t_{i}}}$ where Δt_(i) is the timedifference between two successive exposure clock signal pulses at timet, and α is a constant.
 8. The method of claim 1, wherein the lightoutput function is determined by controlling one or more light sourcesin the printhead using the pulse timing function and using a lightdetector to measure a light output of the one or more light sources as afunction of exposure time.
 9. The method of claim 1, wherein differentpulse timing functions are determined for a plurality of differentprinter configurations.
 10. The method of claim 9, wherein the differentprinter configurations include a first printer configuration having afirst in-track spatial resolution and a second printer configurationhaving a second in-track spatial resolution.
 11. The method of claim 9,wherein the different printer configurations include a first printerconfiguration having a first print speed and a second printerconfiguration having a second print speed.
 12. The method of claim 1,wherein the aim exposure function is modified in response to a printercalibration process, and wherein a new pulse timing function isdetermined corresponding to the modified aim exposure function.
 13. Themethod of claim 1, wherein the pulse timing function specifies a numberof master clock pulses for which the light source should be activated asa function of the pixel code value.
 14. The method of claim 1, whereinan exposure clock signal is formed which includes an exposure clockpulse corresponding to each pixel code value, wherein the pulse timingfor each exposure clock pulse is specified by the pulse timing function.